Windows are conventionally formed by inserting a window assembly into an opening in a wall. The window assembly basically comprises window panes and a sash in which the panes are mounted.
Numerous materials and combinations of materials have been employed to form window sashes. Wood has long been a desirable material for fabricating window sashes; wood is aesthetically pleasing and, being a good insulator, is warm to the touch. However, wood has become quite expensive and is too costly for many window installations. Further, it is difficult to seal the juncture between a wood window assembly and the wall surrounding the opening in which the window assembly is mounted.
Aluminum has also been commonly used as a sash material. Window assemblies using aluminum sashes are relatively inexpensive; however, aluminum sashes have undesirable thermal properties and, under certain weather conditions, "sweat," resulting in moisture accumulating on the inside of the sash.
Extruded polyvinyl chloride (PVC) is also commonly used as a sash material. Extruded PVC sashes are nearly as inexpensive as aluminum sashes and do not sweat like aluminum sashes. However, PVC sashes also have undesirable thermal properties: they are poor insulators and are cool to the touch.
Wood clad with PVC has also been used as a sash material. This product has many of the aesthetic and insulating qualities of an entirely wood sash, and the PVC cladding protects the wood, thereby reducing the long term maintenance costs of the product. However, PVC clad wood sashes are expensive and are still difficult to seal against unwanted air flowing between the window assembly and the frame defining the opening.
Sashes are also made of pultruded fiberglass. Pultruded fiberglass sashes have better thermal characteristics than PVC sashes but not as good as wood or PVC clad wood sashes. Pultruded fiberglass sashes are also expensive and tend to be cool to the touch.